Disk drives typically include a main integrated circuit, which is typically a system-on-a-chip (SOC) that contains many of the electronics and firmware for the drive. The heads are mounted on movable actuators that also contain the preamp IC for the heads. A flex cable connects the SOC to the preamps. The preamps include digital circuitry that includes registers that are set using serial data from the SOC to provide parameters for the preamps functions.
The preamp includes a write driver that generates an analog signal that is applied to the inductive coil in the write head to write data by selectively magnetizing portions of the magnetic material on the surface of the rotating disk. Magnetic transitions representing bits are written into the magnetic material by reversing the current through the write head (writer) coil. Preamp registers can be set to adjust the current magnitude (Iw) and the overshoot amplitude (OSA) and duration (OSD) in order to saturate the media and minimize the magnetic transition (bit) length.
Certain bit patterns present difficulties for write drive signal generation. For example, closely spaced transitions can introduce non-linear bit shift. Conventional disk drives also employ a write-precompensation circuit which compensates for non-linear bit shift. U.S. Pat. No. 6,826,003 to Subrahmanyam (Nov. 30, 2004) describes another approach for disk drive with a pattern dependent overshoot circuit for controlling write current overshoot. Subrahmanyam '003 proposes that a pattern detector can find a particular bit sequence and modify the overshoot amplitude or write current used to write that particular bit sequence. However, comparing the incoming write data with a particular stored sequence at the preamp level will require large preamp overhead (area) and its implementation is challenging. On the other hand, if the pattern detector is implemented in the read/write channel, a digital line between channel and preamp that operates at the channel clock frequency will be necessary to enable/disable the additional overshoot or write current setting. In this case, additional circuitry to perform analog/digital signal addition within a channel clock step must be implemented at the preamp, which is also quite challenging given the prior art. The invention described herein avoids all of these difficulties by replacing the pattern detector with a programmable dynamic wave-shaping write driver.